I'm having the same issue. I use Microsoft Team Foundation Server MSSCCI Provider (version 12.1).
There is no issue when I try to connect via VS2015 or VS2013.
It seems that Ranorex 7 is unable to connect to TFS2015 via this provider.

We would like to take a closer look at this issue. Could you submit a ticket in our support system and mention this post in the ticket? You can use the following form in order to submit a ticket: Support Query